1. Field of the Invention
The present invention refers to a submarine pipeline towing system, equipment and process.
More in particular, the invention is related to techniques for laying pipelines in shallow or very deep waters, in areas which are subjected to currents, through towing said pipelines near to the seabed (off-bottom tow).
2. Description of Related Art
In the oil field it is known to use methods and relative equipment for making marine pipeline sections through towing prefabricated “lines” of pipeline from the manufacturing location to the final site. Amongst the various possible methods the so called “off-bottom tow” is considered as being the most widely-used. The pipeline is towed by making it move slightly above the seabed.
Although this method is used, it has some drawbacks, one of which is that, when there are cross currents, the pipeline can move off-course.
In the patent application WO2006/045357 (U. Giovannini and others) some solutions are mentioned concerning and specifically claiming a technique which is based upon a geometric control of the configuration of the pipeline with respect to the seabed and not to the depth through the use of a series of equipment, connected to the pipeline itself, substantially containing floaters, of which at least a part has a variable buoyancy or partially variable buoyancy which, when placed in a predetermined position and at predetermined intervals, give a graduated buoyancy to the pipeline in function of the distance from the bottom so as to obtain a “festoon-like” configuration of the pipeline, i.e., with suspended pipeline sections, at the floaters, alternating with supporting sections.
The same off-bottom tow technique was cited, as mentioned in the patent application PCT, in the U.S. Pat. No. 4,138,853 (J. E. Lamy), which proposes, in order to counteract the cross current, the use of a device, according to various embodiments, with supporting members which are deformable on the vertical plane but transversally rigid.
The solutions proposed by Giovannini and by Lamy are not without drawbacks. Indeed, although the first solution manages to control the buoyancy of the floaters with respect to the seabed, it is also true that in order to keep the position on course, it relies upon a pipeline/seabed friction force which is sufficient to counteract the force of the cross current. This approach is not without risks due to the unpredictability of the nature of the friction force. On the other hand, the solution of Lamy has, in a complementary way, an approach focused on counteracting the force of the current but it does not consider a specific control of the residual weight of the pipeline, in function of its distance from the seabed, with the risk that, it is not possible to effectively control the configuration of the pipeline during the towing or the residual force that its supporting members discharge onto the seabed, when the operative conditions change. A consequence can be a substantial resistance to the advancing movement. Moreover, neither of these techniques consider any way or any device to bring the towed pipeline back into position, in the case in which this has been deviated by the current (for example in situations of serious adverse sea conditions) i.e., in the cases in which it is necessary to change the course of the towed line (for example to impose a curvature on it).
In the patent application WO2008/017463 (C. Cocca) these drawbacks are solved with a device and a method for the submarine pipeline towing in which the vertical and the horizontal plane configuration control functions are integrated, thanks to the actuation of floaters with a variable or partially variable buoyancy and thanks to guide elements which are able to maintain the towing direction by either counteracting the current force or changing direction. These guide elements interact with the seabed with a partial penetration of a substantially vertical element, the plane of which is capable of rotating on a vertical axis, for example a blade or disc.
The solution described by Cocca solves both the problems concerning the determination of the floaters buoyancy, having only to determine the variation margins of the buoyancy and not a precise value thereof, as well as the uncertainties due to the sliding of the pipeline on the bottom, which now travels parallel to the seabed and not in contact with it.
However, there is still a problem relative to the two previous technologies, which consists in the fact that when there are extreme conditions in the surrounding area (irregularity of the seabed, significant wave motion, etc.) the behaviour of the line could be unstable or in any case difficult to control.